In a flow control system provided with a constriction in the fluid passage, a filter is usually disposed at a position slightly upstream of the constriction so as to inhibit foreign matter from entering the constriction. The foreign matter that might cause clogging of the constriction includes the corrupt substance produced from corrosion of the pipes forming the fluid passage or from degeneration of the fluid itself, and in certain extreme cases, such clogging may be caused by the frayed pieces of fibers constituting the filter. Where the fluid passage is less than 1 mm in diameter, a space tends to be created when a filter is mounted in the fluid passage and the foreign matter might pass through such space to enter the constriction and clog it. This is because the size of the filter itself must be reduced correspondingly to the diameter of the fluid passage and there was available so suitable means for allowing secure airtight coupling of the filter and the fluid passage.